The advent of anisotropic etching processes in silicon integrated circuit technology has reduced feature sizes and allowed higher density packing of circuit elements. Anisotropic etching processes allow openings to be made in dielectric layers, and metalization patterns to be formed, in each case with essential vertical sidewalls, so the size of the openings, and the lateral dimensions of the patterns can be made optimally small, e.g. the minimum line width allowed by the lithography rules. Attending this advance is a drawback. The steep, near vertical, sidewalls are difficult to cover with the the next layer, and they also present sites, sometimes re-entrant sites, where residue remains after a layer has been etched. These problems are well known and widely documented, and a variety of proposals have been put forward for overcoming the "severe step" problem. One of those proposals is to construct a sidewall oxide at the steep edge to smooth the sidewall for the next deposited layer. Such an approach is described and claimed in my copending application Ser. No. 661,614, filed Oct. 17, 1984 and now abandoned. The sidewall oxide is formed by an anisotropic etching process that has become widely used in the so-called "lightly doped drain" (LDD) transistor structure.
I have found that oxide sidewall fillers have limitations in certain applications that prevent their effective use. For example, if the sidewall to be covered has an underlying oxide layer, the prior art oxide sidewall approach involves etching an oxide layer to form the sidewall in the presence of the oxide underlayer, and results in removal of the underlayer unless very precise control is exercised, control that may be impractical in a manufacturing environment. Consequently, when it is necessary to preserve the underlying oxide layer, the oxide sidewall cannot be used effectively. A variety of reasons may arise for retaining that oxide. For example, it is known that exposure of bare silicon to RIE environment can create defects in the silicon that impair device performance. A number of other reasons may arise for creating an alternative approach to sidewall coverage.